The present invention relates to a method and device for correcting a track deviation in an optical disc drive, and particularly to a track deviation correcting method and device for accurately positioning a light spot concentrated by an object lens to a target track by driving a tracking actuator during an optical pick up travel.
The optical disc drive is a device that records and/or reproduces data information that is converted into an optical signal by concentrating a laser beam with a certain size onto an optical disc which is a recording media that is responsive to light. This has the advantages of having a large scale memory capacity that exceeds by tens of times that of the existing magnetic recording type hard disc drive (HDD) and a detachable disk as that of a floppy disc drive (FDD), thereby its use is broadened as an auxiliary memory device for a computer. However, in order to sufficiently serve as a computer memory device, an access time similar to that of a HDD is required.
Track access of the optical disc drive is composed of a coarse seek stage and a fine seek stage. The coarse seek, for example, means a stage to transfer an optical pick up to a target track by operating a pick up transfer such as a voice coil motor (VCM). However, unlike the HDD, because track deviation occurs from wobble due to an inferior disc assembly, a light spot formed in the optical pick up cannot match accurately the target track by the coarse seek alone.
Therefore, a position correction with respect to the track deviation must be performed by a fine seek operation rather than a course seek operation. The fine seek means a stage to minutely displace the object lens forming the light spot by driving a separate tracking actuator. This operation continues so that the light spot follows up the track center even after completion of the seek operation.
In order to correct a track deviation that occurs from the disc wobble, the fine seek operation is generally executed after the coarse seek operation is completed. However, this lengthens the overall seek time and the access time which is the seek time plus the disc rotation waiting time, thereby making the realization of high speed access difficult. Accordingly, in order to attain high speed access, there is a requirement to execute the fine seek operation in parallel with the coarse seek stage to correct track deviation so that the light spot is brought closer to the target track position as much as possible by the object lens immediately after the coarse seek has been completed.
Among such conventional methods, there are an actuator deviation synchronizing drive method, a target position information analyzing method and a track counting method.
The actuator deviation synchronizing drive method uses an optic sensor that is installed separately from an optical pick up to detect the amount of disc deviation and feedback the signal detected by the optic sensor to the actuator and drives the actuator in synchronous with the wobble occurring due to the disc deviation. In this method, in order to detect the track cross signal of the disc by means of the optical sensor, the optical sensor must at least be of performance similar to that of the object lens of the light pick up. Accordingly, its unit cost becomes high.
The target position information analyzing method is a method that detects the amount of track deviation in the light pick up and stores it in a memory device, and drives the actuator by a correction signal for a periodical track deviation. This method is sensitive to disc wobble, which reduces its degree of accuracy.
The track counting method is a method that counts a track number of a disc where a light spot crosses in the optic pick up during a seek operation, increases or decreases speed of the VCM used in the course seek operation and transfers controllably the optical pick-up to the target track. This method has the advantage of having the highest degree of accuracy when compared with the above two methods but it is sensitive to disc wobble. This results in frequent errors and a complicated circuit.
In the mean time, in order to load the light spot on the track of the disc, control of a tracking actuator becomes possible when the mutual relative speed between the light spot and the track moving in a seek direction falls below a specified value. However, in the above mentioned conventional methods, the relative speeds are significantly greater because the tracking actuators are all driven by taking a VCM drive signal as a reference signal. In the conventional method, therefore, the speed of the VCM must be reduced in the vicinity of the spot of the course seek completion to allow for the position control time between the course seek and fine seek operations. This position control time generally takes about 5 msec and functions as an obstacle factor to realize a high speed.